A closure selectively closes an access port on a pressure-containing vessel, providing access to its interior. One of the more frequent applications for a closure is on pipelines to launch and retrieve a “pig,” which is a device used in cleaning and inspecting pipelines. Historically most operations have used a working pressure below 5000 psi. Recently, sectors of the petroleum industry are requiring the use of higher pressure closures.
Closure designs commonly fall into one of three groups: threaded style, clamp style, or flat door. Each style has 3 basic parts: 1) a hub section permanently and sealingly secured to the vessel or pipeline, 2) a door which provides a seal against the hub and 3) a door hinge or transport mechanism.
The threaded style closure can be cumbersome to operate. Threading a cap onto a hub increases in difficulty as the diameter increases. Even a modest size cap is commonly articulated to the hub by a crane or davit-type hinge arrangement. Corrosion on the threads or seal surfaces can make operation difficult or, in severe cases, impossible. Yet, threaded style closures are popular because they offer a simple and safe design, as demonstrated by the large number in actual field use. U.S. Pat. No. 6,685,380 discloses an example of a threaded type closure having a chain binder for rotating the closure to thread or unthread it. The chain binder is an alternative to conventional hammer lugs, which must be struck with a hammer to close the closure. U.S. Pat. No. 6,663,149 discloses another alternative to hammer lugs, wherein a tightening ear is used to apply a torque to the closure cap. U.S. Pat. Nos. 6,286,553 and 3,667,647, U.S. Patent Application 2004-004081-A1, GB 2132956, and European Specification 0129349 disclose variations of a flat door closure. The ASME has published a figure showing “Some Acceptable Types of Unstayed Flat Heads and Covers,” which shows a diagram of a conceptual closure that includes a locking ring threadedly secured to a hub and structurally separate from a door. The flat door version offers the quickest access of the three types. Yet, its weight makes installation difficult and corrosion over long periods of time can inhibit operation. Elaborate sealing techniques are also required. The flat door is usually secured to the hub with a self hinging or lifting arrangement.
The clamp style closure functions by bringing two flanges together and then securing their position with an external annular locking ring arrangement. Commonly, the annular lock ring is presented in two segments which may be held together via a bolting arrangement. While the concept is simple, the closure is heavy, and difficult to operate and seal. Not only does the door require some handling/articulation to the permanent portion of the closure, but the locking ring itself poses handling difficulties. Some type of powered drive is commonly required to secure such a device. One major inhibition is that the presence of pressure is not obvious to the operator. Cases of catastrophic failure and doors being opened under operating pressure have resulted in death and injury. A clamp style closure is disclosed in U.S. Pat. No. 6,125,888.
Although a number of different closures are in use, few of them, if any, are designed for operation at higher pressure levels more frequently demanded by the petroleum industry. A higher pressure and increasingly reliable closure is therefore desirable.